Description of Prior Art
a. Conventional Copper Smelting. PA0 b. Conventional Copper Converting. PA0 c. Flash Smelting Techniques. PA0 Fundamental Steps In Copper Smelting and Converting PA0 Continuous Copper Smelting Processes
The smelting of copper sulfide concentrates has been carried out primarily in reverberatory or electric furnaces. Copper concentrates have been charged in those furnaces either wet or after roasting.
Energy consumption is very significant in these furnaces. Energy is required for heating the charge and supplying the necessary latent heat of fusion to obtain the molten phases of slag and matte. In the reverberatory furnaces, a large volume of combustion gas is produced which, along with the air infiltration into the furnace, leads to a very severe dilution of sulfur dioxide (SO.sub.2) produced during smelting. Because a significant fraction of the sulfur in the copper sulfide containing charge (20-35%, by weight on elemental basis of sulfur, for green charge furnaces) is removed during smelting, the consequent production of very large volume of gas, with a low SO.sub.2 concentration, makes any attempt to control this diluted sulfur emission very expensive.
A molten matte is mainly composed of copper and iron sulfides. It is transported from the smelting furnace in ladles (by cranes) to P-S (Peirce-Smith) horizontal converters. Air is blown in the P-S converters and, thus, iron is oxidized and removed in a slag phase, whereas sulfur is oxidized to gaseous sulfur dioxide (SO.sub.2). The P-S converters are cylindrical furnaces, up to 35 ft. long and with diameter up to 15 ft. A "mouth" at mid-length of the P-S converter serves as a gas exit. Through this "mouth" these furnaces are charged and discharged. During operation, the P-S converter mouth is under a water-cooled hood which collects the SO.sub.2 -containing gas. As the hood is under a slight negative draft, a significant air infiltration occurs thus diluting strongly the gaseous converter product. This dilution may be reduced at a lower draft, but part of the SO.sub.2 -containing gas escapes from the hood and pollutes the plant environment.
Fugitive emissions of SO.sub.2 gas also occur during the transportation of matte and during charging and discharging of the P-S converter. Secondary hoods--expensive to install and operate--have been employed recently for the collection of these fugitive emissions. Notwithstanding the cost of secondary hoods, the collection of the escaping SO.sub.2 gas is often unsatisfactory.
The converting of copper matte is a batch operation divided in practice in two stages. Stage one is "converting-for-slag," that is removal of iron and partial removal of sulfur. Stage two is "converting-for-copper," that is completion of sulfur removal from copper and production of blister copper.
During the first stage of converting, iron is oxidized and iron oxides with silica flux form mostly molten silicates of relatively low viscosity.
This "converting-for-slag" stage is composed of several cycles (e.g. 4 to 9, but typically 6 to 8 cycles). At the end of each cycle, slag is discharged, a new quantity of matte is charged, and the converting starts again. Fugitive emissions of SO.sub.2 occur during each cycle, and especially during discharging and charging the converter. After the removal of iron, the enriched matte (white metal, about 79% Cu) is converted to blister copper.
New copper smelting plants have adopted flash smelting techniques. A dry charge is blown into the flash furnaces, together with preheated air or oxygen-enriched air or oxygen, to form a suspension of sulfide (and flux) particles within the oxidizing gas medium. Roasting, smelting and partial converting reactions are taking place at an extremely rapid rate. Flash smelting can be autothermal if appropriately adjusted flow rates of oxygen are used. Flash smelting process yields a high grade of matte. A matte, produced in the flash smelting furnace, nevertheless, has to be transported to the converters and oxidized to obtain blister copper employing the same two-stage, multi-cycle, batch operation as in the conventional process.
The slag produced in flash smelting is usually highly oxidized and has high copper content. This slag has to be treated separately for copper recovery therefrom. Flash smelting, in spite of its significant advantages over conventional smelting, has a number of drawbacks.
One drawback is that flash smelting is a multi-step operation with molten sulfides, slags, and blister copper transported by ladle and crane from furnace to furnace.
Another drawback is that periodic tapping of (high grade) matte from the flash smelting furnace is required; this and transportation of the matte to the converters cause fugitive emissions of SO.sub.2 gases.
Another drawback of flash smelting is that it still requires the batch operation of P-S converters. The last contributes strongly to fugitive SO.sub.2 gas emissions.
A smelter employing flash smelting furnaces has two sources of high concentration SO.sub.2 gases, one from the flash smelting furnace and the other from the converters. These gases are often the feed material for an on-site sulfuric acid plant. However, the fluctuations of the converter gas, both in flow rate and SO.sub.2 concentration, restrict the efficiency of the acid plant and thus operates as another drawback.
Starting with copper sulfide concentrates, the pyrometallurgical production of copper is a progressive and controlled oxidation reaction. The activity of oxygen (i.e. partial pressure of oxygen in the system, or expressed otherwise-concentration), in the production system, is gradually increased during smelting and converting. Conventional smelters, as well as flash smelters, have produced millions of tons of copper by following three distinct consecutive steps (in separate furnaces):
1. Smelting (at low oxygen activity), PA1 2. Converting-for-slag (high oxygen activity), PA1 3. Converting-for-copper (high oxygen activity in the absence of iron compounds).
The thermodynamic equilibria of the simultaneous oxidations, which take place during copper smelting and converting, require the three-step operation and indicate the conditions that must be respected during pyrometallurgical copper making. These conditions are illustrated in FIG. 11 and will be further discussed. An attempt to oxidize copper sulfide, in the presence of slagged iron, results in the oxidation of iron to magnetite and copper ferrites. A high content of magnetite and copper ferrites in the slag gives a viscous, or quasi-solid slag, with extremely high copper content. This type of slag impedes an efficient production of copper.
Contaminants--such as arsenic, antimony, bismuth are usually found in copper concentrates. A significant proportion of these contaminants dissolves in the matte. If matte contaminated with As, Sb, Bi is converted in the presence of molten metallic copper, those contaminants tend to dissolve in the metal (causing detrimental complications during its subsequent refining). When such contaminated matte is converted in the absence of a metallic phase--as in the stepwise converting--the impurities are oxidized and mixed in the slag phase.
Three continuous copper smelting processes have been tried on a pilot plant scale and two of those are currently in operation. These processes are known as the WORCRA, the NORANDA, and the MITSUBISHI process.
In a WORCRA process, it is suggested to perform smelting and converting in a single furnace, with the matte and slag flowing countercurrently. There is no attempt to partition the furnace into distinct smelting and converting zones. The WORCRA process, after long pilot plant testing, failed to develop as an industrial process.
The NORANDA process proposed the continuous production of blister copper and rejectable slag in a cylindrical furnace equipped with tuyeres (similar to an elongated and modified P-S converter). The proposed reactor is indicated as composed of three zones (smelting, converting and slag cleaning), but without any distinct partition between these zones and under a common gas space throughout. Industrial tests failed to produce a "clean" rejectable slag. A viscous slag, high in magnetite and copper contents, was produced. This slag required further treatment outside the reactor. In addition, concentrates contaminated with As, Sb, and Bi yielded blister copper containing these contaminants, thus causing difficulties in the subsequent refining of the metal.
Consequently, the NORANDA process, as operated industrially, is not a continuous copper-making process. The NORANDA reactor is a smelting furnace producing high grade matte (to be converted) and slag with very high copper content (to be treated in an additional operation).
The third process, known as the MITSUBISHI process, employs three interconnected furnaces. In this process smelting is distinctly separated from converting, thus single stage converting is employed, i.e. in a separate furnace, in the presence of molten copper phase. However, the three-furnace concept maximizes heat losses. Further, the movement of molten materials from furnace to furnace leads to fugitive emissions of SO.sub.2 gas.
For today's copper production, a clean environment with low energy consumption is a desideratum. The increasingly stricter regulations for controlling sulfur emissions and for operating environmentally "clean" plants require the development of a continuous copper smelting and converting process in a single furnace, with a single source of effluent SO.sub.2 gas. The high cost of energy is a strong incentive for the development of an autothermal process (utilizing the heat of oxidation of iron and sulfur) within a single furnace and for the production of a low volume of gas (with high SO.sub.2 content).